Semiconductor light emitting diodes (LEDs) have replaced conventional incandescent, fluorescent and halogen lighting sources in many applications due to their small size, reliability, relatively inexpensive cost, long life and compatibility with other solid state devices. In a conventional LED, an N-type gallium arsenide substrate that is properly doped and joined with a P-type anode will emit light in visible and infrared wavelengths under a forward voltage bias. In general, the brightness of the light given off by an LED is contingent upon the number of photons that are released by the recombination of electrons and carriers inside the LED semiconductor material. The higher the forward voltage bias, the larger the current, and the larger the number photons are emitted. Therefore, the brightness of an LED can be increased by increasing the forward voltage. However due to various limitations, including the ability to dissipate heat, conventional LEDs have, until recently, been capable of producing only about six to seven lumens.
In the past few years, advanced High Power LEDs, alternately known as High Brightness LEDs (HB-LEDs), have been developed which demonstrate higher luminosity, lower heat profiles, and smaller footprints enabling the use of multiple LEDs in composite area lighting systems. The Cree X-Lamp XR-E, as an example, can produce 136 lumens of luminous flux at 700 mA, with a forward voltage of 3.5V. Its thermal design provides a ratio between the resistance junction and ambient temperature of as low as 13° C./W at maximum current. It provides a small footprint (4.3×7.0×9 mm). They are also reflow-solderable, using a thermal ramp scheme with a 260° C. maximum, enabling certain applications germane to the present invention. Comparable competitive LED products are only slightly behind in market introduction, such as Seoul's Star LED and Luxeon's “Rebel” High Power LEDs.
High-power LEDs still suffer from problems associated with heat dissipation and inefficient distribution of light for certain applications. While high-power LEDs are significantly more efficient than incandescent systems or gas-filled (halogen or fluorescent) systems, they still dissipate on the order of 50% of their energy in heat. If this heat is not managed, it can induce thermal-runaway conditions within the LED, resulting in its failure. For situations requiring high levels of lighting, this situation is aggravated by the requirement of combining many LEDs in a sophisticated composite light-source structure such as an underwater lighting fixture. Heat management becomes a primary constraint for applications seeking to use the other advantages of LEDs as a source of illumination.